CHARLESTON, W.Va. (AP) — West Virginia's mine safety chief says he's begun issuing subpoenas to reluctant witnesses to the nation's worst coal mining disaster in 40 years.

As of Friday, Ron Wooten says he's signed subpoenas for five people. All skipped interviews with investigators probing the April 5 explosion at Virginia-based Massey Energy's Upper Big Branch mine.

Twenty-nine men died in the blast. Investigators suspect methane gas and coal dust, but have not established a definite cause.

West Virginia rarely exercises subpoena power in mine investigations, but Wooten says investigators really need to speak with the people who've been subpoenaed. The state is working with the federal Mine Safety and Health Administration, which can only compel witnesses to appear at formal public hearings.